Correct me if I'm wrong, but build.py does not appear to implement ue4-ci-helpers

This is particularly awkward because ue4-opencv-demo is linked from the ue4-docker documentation under the heading "Usage with Continuous Integration systems."

It is somewhat confusing that ue4-ci-helpers is not being implemented in the demo that is linked to under the guise of CI. Additionally, it makes understanding the architecture and intended usage of the awesome tooling provided by @adamrehn more difficult.

Once I get the demo running I can attempt to port build.py and open a PR, but of course having it provided by the expert would definitely be appreciated 😄